Referring to FIG. 14, a schematic diagram of a conventional heat pump refrigeration system for a building is shown. The heat pump refrigeration system includes refrigerant compressor 1 which is driven by engine 2 through power transmission 3, e.g., a V-shaped belt. Engine 2 is typically a single speed engine or a multi-speed engine which operates at several fixed speeds in order to adjust the capacity of the compressor to which it is connected. The heat pump refrigeration system further includes four-way valve 4, first heat exchanger 5 which is located outside a refrigeration compartment, and second heat exchanger 6 which is located inside the refrigeration compartment. Outlet port 11 of compressor 1 is connected to inlet port 41 of four-way valve 4. Although not shown in FIG. 14, it is conventional to mount a thermal sensor adjacent second heat exchanger 6, which operates as an evaporator during a refrigerating cycle, to sense the amount of frost accumulating on the second heat exchanger by measuring its temperature. When the temperature decreases below a predetermined temperature, the heat pump refrigeration system switches to a defrosting cycle. This is accomplished by a conventional electrical circuit (not shown) which compares the voltage on the thermal sensor with a predetermined voltage, and generates an electrical signal which actuates four-way valve 4 when the temperature falls below the predetermined temperature. This electrical circuit also turns off fan 14 during the defrosting cycle. Thus, four-way valve 4 selectively switches between a refrigerating cycle and a defrosting cycle as illustrated by FIG. 14 and described in further detail below. In FIG. 14, first connecting port 42 of four-way valve 4 is connected to one end of first heat exchanger 5. The other end of first heat exchanger 5 is coupled with one end of second heat exchanger 6 through first check valve 7, first expansion valve 8, second check valve 9 and second expansion valve 10. The other end of second heat exchanger 6 is connected to second connecting port 43 of four-way valve 4. Third connecting port 44 of four-way valve 4 is connected to inlet port 12 of compressor 1.
In the refrigerating cycle, refrigerant flows in the refrigeration circuit in the direction shown by solid-line arrows in FIG. 14. Refrigerant which is discharged from compressor 1 flows into one end of first heat exchanger 5 through first connecting port 42 of four-way valve 4. Gaseous refrigerant at high temperature and pressure is condensed and heat is removed from the refrigerant at first heat exchanger 5. Refrigerant then flows from the other end of first heat exchanger 5 to first expansion valve 8 through first check valve 7. The refrigerant expands at first expansion valve 8 and flows into one end of second heat exchanger 6. The refrigerant is vaporized at second heat exchanger 6, and the refrigerant absorbs the surrounding heat to cool the refrigeration compartment. Thereafter, the refrigerant returns to inlet port 12 of compressor 1 through second and third connecting ports 43 and 44 of four-way valve 4. Accordingly, in the refrigerating cycle, first heat exchanger 5 is used as a condenser and second heat exchanger 6 is used as an evaporator.
On the other hand, in the defrosting cycle, refrigerant flows in the refrigeration circuit in the direction shown by dotted-line arrows. Gaseous refrigerant at high temperature and pressure discharged from outlet port 11 of compressor 1 flows into second heat exchanger 6 in a direction opposite to the direction of flow in the refrigerating cycle. Refrigerant flows through second connecting port 43 of four-way valve 4 to second heat exchanger 6. Gaseous refrigerant is condensed and heat is removed from the refrigerant at second heat exchanger 6, and the refrigerant changes into liquid refrigerant at high pressure. At this time, frost on the outer surface of second heat exchanger 6 in the refrigeration compartment melts into a drain pan (not shown). The water collected in the drain pan is removed through a drain hose (not shown). Liquid refrigerant at high pressure flows from second heat exchanger 6 through second check valve 9 to second expansion valve 10 where the pressure of the liquid refrigerant is lowered. The liquid refrigerant then flows into first heat exchanger 5 where it absorbs the surrounding heat and changes into a gaseous refrigerant. The gaseous refrigerant returns to inlet port 12 of compressor 1 through first and third connecting ports 42 and 44 of four-way valve 4. The gaseous refrigerant is compressed and again discharged by compressor 1. The above operation is repeated until the defrost on second heat exchanger 6 is removed. Accordingly, in the defrosting cycle, unlike the refrigerating cycle, first heat exchanger 5 is used as an evaporator and second heat exchanger 6 is used as a condenser.
One of the disadvantages of the above described conventional heat pump refrigeration system is that, in the event ambient temperature is high, the temperature of the refrigerant discharged from compressor 1 becomes extremely high in both the refrigerating and defrosting cycles. The temperature of the discharged refrigerant also may increase in multi-speed heat pump refrigeration systems when the system is operated at a high speed setting. Temperature increases may also occur due to transfer of heat between discharged and suctioned refrigerant at four-way valve 4, and heat conduction from engine 2 to compressor 1. Thus, compressor 1 may overheat, and it is possible that compressor 1 or a refrigerant hose connecting compressor 1 and four-way valve 4 will be damaged.
Another disadvantage of the above described conventional heat pump refrigeration system is that only partial defrosting may be accomplished during the defrosting cycle. When water drops on the drain pan, it may be refrigerated during the refrigerating cycle. If so, it is not necessary to melt the frost on second heat exchanger 6 during the defrosting cycle but also on the drain pan. In the conventional system, during the defrosting cycle, discharged gas refrigerant at high temperature and pressure is passed through a conduit mounted on the drain pan after it passes through second heat exchanger 6. However, when the refrigerant reaches the drain pan, its temperature has decreased and the capacity for defrosting is reduced. Water then freezes on the bottom portion of the drain pan and around the water discharge hole formed on the drain pan.
A further disadvantage of the conventional heat pump refrigeration system of FIG. 14 is that it is not readily adaptable for use on a motor vehicle with the compressor driven by the engine of the motor vehicle. Even though heat pump refrigeration systems are generally more efficient and have an improved defrosting cycle, such systems apparently have not been installed on motor vehicles and driven by the motor vehicle engine because of the continuously variable speed of such engines. If the compressor of a heat pump refrigeration system is driven by a continuously variable speed engine, the speed of the compressor also changes continuously. At high engine speed, which results in high compressor speed, the temperature of the refrigerant discharged from the compressor increases considerably which reduces the efficiency of the operation of both the defrosting and refrigerating cycles of the refrigeration system.